Down on HP and up on weight
Van tried the 6 banger Franklin in a RV-8. They found it's claimed HP was inflated, performance was down from a 4 cyl Lyc. There was also a weight penalty. You may have a hard time finding parts or parts at reasonable price. A lyc has many vendors for every part, not to mention the previous post, the plane was designed for a Lyc. Going your own way will add 100's of hours of work and hunting for parts, (engine mount, baffles, exhaust and on and on). Of course the Lyc has more options like electronic ignition. I don't know much about the little 4-cyl Franklin but I am sure it works, but in light of the draw backs I would almost have to get the engine free to use it.